The present invention relates to novel cationic polymeric coatings and plastic substrates comprising such coatings, as well as methods for preparing same.
Typically, in the production of emulsion polymers by free-radical polymerization, it has been a common practice to use nonionic or anionic materials to stabilize the emulsions. Alternatively, others have used protective colloids like poly(vinyl alcohol) [PVOH], hydroxyethyl cellulose, or derivatives thereof, either singly or in combination, to stabilize aqueous emulsion polymers. These stabilizers produce emulsion particles that are electrically neutral or negatively charged in an aqueous environment.
Commercially produced emulsions that contain positively charged particles (that is, cationic emulsions) are known, but are far fewer in number. U.S. Pat. No. 5,521,266 to Lau discloses the synthesis of cationic emulsions, but requires complexation of water-insoluble monomers with macromolecular carbohydrates (for example, cyclodextrin, cyclodextrin derivative, cycloinulohexose, cycloinuloheptose, cycloinuloctose, calyxarene and cavitand) having a hydrophobic cavity.
U.S. Pat. No. 4,308,189 to Moritani et al. reviews many conventional techniques for producing cationic emulsions using low-molecular-weight cationic emulsifiers such as laurylamine salt, octadecylamine salt, laurylpyridinium chloride, and others that are toxic and subject to stringent regulations. The ""189 patent also describes the use of cationic initiators and monomers to create cationic emulsions. Moreover, one can add materials like alkylaminopolyoxyethylene to nonionic and anionic emulsions to render emulsion particles cationic. Furthermore, the ""189 patent unfavorably described the potential for using protective colloids made from cationic derivatives of carbohydrates (for example, starch and cellulose) and polyamide-polyamine epichlorohydrin, which according to U.S. Pat. No. 2,926,154 to Keim is the reaction product of epichlorohydrin with polyamides containing the following recurring groups:
xe2x80x94NH(CnH2nHN)xxe2x80x94COR3COxe2x80x94
where n and x are each 2 or more and R3 is the divalent organic radical of a dicarboxylic acid. According to Moritani et al., these materials are poor stabilizers and usually used in combination with nonionic surfactants.
Moritani et al. disclosed an improved class of protective colloids based on cationic group-modified PVOH. While these materials improve fixation affinity for pulp and improved bonding affinity for glass, sand, and concrete, they are not suggested to disclose improved adhesion to plastic film, especially after prolonged exposure to water. Indeed, PVOH and its water-soluble derivatives are prone to swell and release from plastic upon prolonged exposure to water.
U.S. Pat. No. 5,254,631 to Yamamoto et al. discloses cationically electrodepositable, finely divided gelled polymers (that is, internally cross-linked polymer particles that do not coalesce into a film upon drying) having a core-sheath structure obtained by emulsion polymerization. They used water-soluble or water-dispersible cationic resins as a stabilizer in their emulsion-polymerization process. The resin used in their process has, in the molecule, a cationic functional group capable of imparting positive charges and hydrophilicity when neutralized with an acid. They created this water-soluble cationic resin by by reacting a polyphenol with epichlorohydrin to form a polyglycidyl compound compound having a number-average molecular weight between 800 and 2000 and then reacting the epoxy group of the polyglycidyl compound with a cationizing agent.
The polyphenols that they described include bis(4-hydroxyphenyl)-2,2-propane, 4,4xe2x80x2-dihydroxybenzophenone, bis(4-hydroxyphenyl)-1,1-ethane, bis(4-hydroxy-tertbutylphenyl-2,2-propane, bis(2-hydroxynaphthyl)methane, 1,5-dihydroxynaphthalene, bis(2,4-dihydroxyphenyl)methane, tetra(4-hydroxyphenyl)-1,1,2,2-ethane, 4,4xe2x80x2-dihydroxydiphenyl ether, 4,4xe2x80x2-dihydroxydiphenylsulfone, phenol novolac and cresol novolac.
They selected cationizing agents from a list of relatively small amine-containing molecules such as ammonia, hydroxylamine, hydrazine, hydroxyethylhydrazine, N-hydroxyethylimidazoline compound and
(1) Primary amines such as methylamine, ethylamine, n- or isopropylamine, monoethanolamine, n- or isopropanolamine and the like.
(2) Secondary amines such as diethylamine, diethanolamine, di-n- or di-isopropanolamine, N-methylethanolamine, N-ethylethanoilamine and the like.
(3) Polyamines such as ethylenediamine, diethylenetriamine, hydroxyethylaminoethylamine, ethylaminoethylamine, methylaminopropylamine, dimethylaminoethylamine, dimethylaminopropylamine and the like.
Therefore the number-average molecular weight of the water-soluble cationic resin will not be significantly more than 2,000 and probably less than about 3,000, especially since Yamamoto et al. teach that content of the cationic group in their water-soluble or water-dispersible cationic resin should be kept as low as possible.
So, there are few processes that yield stable emulsions and none that offer satisfactory performance properties in applications that require film formation on and adhesion to plastic film or adhesion to inks after prolonged exposure to water or solvents like isopropyl alcohol (IPA).
For example, U.S. Pat. No. 4,214,039 to Steiner et al. discloses a cationic polymer as a primer for vinylidene chloride polymers used as coatings for oriented polypropylene packaging films. The primer comprises an epoxy resin composition comprising a) a liquid epoxy resin, e.g., one based on Bisphenol A, preferably emulsified or dissolved in water, and b) a water-soluble, amine-modified acrylic resin. This system, also employed at higher coating weights in U.S. Pat. No. 6,025,059 to McGee et al., has limited shelf stability. Once the ingredients are mixed, the ingredients start to react. The useful pot life of the mixture of the ""059 patent is no more than about 3 days. After this, the mixture gels or agglomerates, with precipitation of components. Moreover, undesired blocking can occur at coating weights below 0.25 grams/1000 in2 (g/msi). In addition, ink adhesion problems can occur during printing with black UV-curable screen ink. Finally, the formulation may contain amounts of up to 10-20 wt. % propylene glycol monomethyl ether, which may require certain precautions in handling and use on a commercial scale.
Typically, films prepared for use as label facestock are coated on the printing side with a coating, which enhances ink adhesion. For instance, U.S. Pat. No. 5,380,587 to Musclow et al. discloses a multilayer packaging or label stock film having excellent printability and non-blocking characteristics. The film is first primed and then coated with copolyester coating.
Another ink adhesion enhancing coating is described in U.S. Pat. No. 5,382,473 to Musclow et al. which discloses a multilayer film structure with a prime coating which is the reaction product of acidified aminoethylated vinyl polymer and epoxy resin, top coated with polyester ink base to eliminate blocking.
U.S. Pat. No. 5,662,985 to Jensen et al. discloses a two-side coated label which comprises a polymeric film substrate having on a first surface thereof (A) an adhesive anchor layer and on a second surface thereof (B) an ink base layer, the (A) and (B) layers being selected from the group consisting of: (i) a prime coating having on an external surface a functional coating of an interpolymer of (a) an alpha, beta-monoethylenically unsaturated carboxylic acid; and (b) a neutral monomer ester comprising an alkyl acrylate ester and an alkyl methacrylate ester; and (ii) an iminated polymer; or the (A) adhesive anchor layer being selected from the group consisting of: (iii) a mixture of the functional coating of (i) and the iminated polymer of (ii); (iv) a linear water dissipatable polyester condensation product; and (v) a polyester; or the (B) ink base layer being selected from the group consisting of: (vi) a prime coating having on an external surface a functional coating of an acrylic copolymer; and (vii) a prime coating having on an external surface a functional coating of a styrene copolymer, provided that each of the (A) adhesive anchor layer and the (B) ink base layer is different. This invention offers excellent adhesion to most inks, but lacks resistance to IPA and blushes when the coated film is exposed to hot water.
U.S. Pat. No. 5,089,335 to Patton et al. teaches a cross-linking primer for flexible packaging film which is a copolymer of one or more acrylic comonomers and a cross-linking copolymerizable co-monomer having pendant free hydroxyl groups or groups convertible to free hydroxyl groups. The primer improves adhesion of a poly(vinylidene chloride) [PVdC] moisture- and oxygen-barrier topcoat to the substrate.
One-package aqueous latices containing alkaline-curable self-cross-linking polymers are disclosed in U.S. Pat. No. 4,546,140 to Shih. These cationic polymer emulsions were stabilized by cationic monomers and nonionic surfactants. Shih""s invention requires the presence of a salt of an organic carboxylic acid to cure polymerized acrylic esters containing a halohydrin and/or a quaternary ammonium salt. Shih""s emulsions do not contain epoxy-functional monomers.
All of the foregoing U.S. patents are incorporated herein by reference.
The development of commercially acceptable coated plastic films for printing applications, e.g., printable labels, is often a compromise between a variety of desired properties. Labels used for beverage containers, or health and beauty containers, should be capable of exposure to severe conditions encountered during manufacturing, transport and storage. Thus printable coatings for plastic films should exhibit hot-water resistance, organic-solvent resistance, e.g., IPA resistance, abrasion resistance, and haze resistance on exposure to hot or cold water. At the same time, the coating should be receptive to ink so as to provide good adhesion of the ink to the coated film immediately after printing. The ink should stay adhered to the coated film after a label is made and applied to a beverage container that is exposed to hot or cold water and subsequent abrasion encountered in mechanized handling.
Non-cross-linked polymer constituents of coatings tend to increase in haze upon exposure to boiling water and may be completely soluble and removed upon exposure to IPA. In addition, after ink is applied and the label exposed to water and abrasion simulating a bottling line, the ink will abrade off the label. Coatings can be made resistant to hot water or chemicals by cross-linking polymers in the coating. However, when cross-linked, coatings are generally less receptive to inks, losing the ability to have good ink adhesion immediately after printing, especially at high printing speeds and low temperature.
It would be highly desirable to provide stable cationic emulsions that allow sufficient design flexibility to create useful coatings for plastic film, such as acrylic or PVdC coatings that offer barrier to aromas, moisture, and oxygen. In particular a coating composition that contains both epoxy and hardener components in a single emulsion with a longer pot life than 3 days, e.g., several weeks. It would also be desirable that such a coating composition exhibits a chemical-resistant, printable surface such that the coated product can be used as a primer for other coatings (cationic, nonionic, or anionic) or as a label that exhibits both acceptable solvent resistance properties and ink adhesion properties, particularly adhesion to UV-curable screen inks. Moreover, it would be desirable that such a coating composition resists blocking at low coating weights, which are economically favorable. Furthermore, it would be desirable to have stable cationic emulsion polymers that could be formulated with additives to enhance adhesion to particular substrates or inks or to impart color, texture (a matte finish or paper-like appearance), anti-static properties, and/or security features.
In one aspect, the present invention relates to a cationically stabilized emulsion polymer comprising a combination of at least one polymerizable monomer which is uncharged or positively charged in an aqueous solution having a pH between 1 and 8, polymerized in the presence of at least one water-soluble polymer having a number-average molecular weight greater than 5000 which comprises a moiety selected from the group consisting of primary amines, secondary amines, tertiary amines and quaternary ammonium salts, with less than 5 weight percent, preferably less than 1 weight percent, of the monomer units in the water-soluble polymer being comprised of copolymeric units derived from at least one member of the group consisting of carbohydrates, modified carbohydrates, and units having the following formula: 
wherein R1 is selected from the group consisting of H, C1 to C6 alkyl, and C1 to C6 acyl and R2 is selected from the group consisting of H, C1 to C6 alkyl, and the reaction product of epichlorohydrin with polyamides containing the following recurring groups:
xe2x80x94NH(CnH2nHN)xxe2x80x94COR3COxe2x80x94
where n and x are each 2 or more and R3 is a divalent organic radical of a dicarboxylic acid.
In another aspect, the present invention relates to a cationically stabilized emulsion polymer that comprises on a dry basis: i) 30 to 97 wt. % of at least one vinylic, non-acidic monomer which is uncharged or positively charged in an aqueous solution having a pH between 1 and 8; and ii) 3 to 70 wt. % of at least one water-soluble polymeric compound having a number-average molecular weight greater than 5000 which comprises a moiety selected from the group consisting of primary amines, secondary amines, tertiary amines and quaternary ammonium salts.
In still yet another aspect, the present invention relates to a plastic film that comprises i) a plastic substrate and ii) the above-described dried emulsion coating composition on at least one side of said substrate.
In another aspect, the present invention relates to a plastic film which comprises A) a plastic substrate layer, B) a primer layer containing a cationically stabilized emulsion polymer that comprises on a dry basis: i) 30 to 97 wt. % of at least one vinylic, non-acidic monomer which is uncharged or positively charged in an aqueous solution having a pH between 1 and 8; and ii) 3 to 70 wt. % of at least one water-soluble polymeric compound having a number-average molecular weight greater than 5000 which comprises a moiety selected from the group consisting of primary amines, secondary amines, tertiary amines and quaternary ammonium salts; and C) a coating layer which imparts properties to the film which properties are selected from moisture barrier, gas barrier, light barrier, printability, receipt of an image via a process selected from the group consisting of electronic, magnetic, thermal, and photographic processes, generation of an image via a process selected from the group consisting of electronic, magnetic, thermal, and photographic processes, carrying of anti-microbial agents, carrying of colorants, carrying of scents, water absorption, organic solvent absorption, release from pressure-sensitive adhesives, mar resistance, anti-static, conductivity, machinability, sealability, and adhesion.
For present purposes, the term xe2x80x9ccationically stabilized emulsionxe2x80x9d relates to emulsions containing a polymer having positive charges along its backbone, which are generally associated with negatively charged counterions like Clxe2x88x92, Brxe2x88x92, NO3xe2x88x92, SO4xe2x88x922, RCO2xe2x88x92 derived from inorganic or organic acids of relatively low molecular weight. However, where such positively charged polymers are mixed with another polymer having anions on the polymer backbone, the two polymers will coagulate. Moreover, if the localized pH around the cationic polymer exceeds 8.0, xe2x80x9ckick outxe2x80x9d or coagulation of the polymer will occur. Accordingly, it is important that these materials be prepared in an environment that minimizes exposure to anionic polymer.
Plastic Substrate
The plastic substrate to be coated can be any thermoplastic material. Preferably, the plastic substrate is produced from a thermoplastic material, such as polyolefins, polyamides, polyesters, and polyethylene terephthalate. Examples of polyolefins to be used include alpha-olefins produced by Ziegler-Natta or metallocene catalysts, such as polyethylene, polypropylene, and copolymers and terpolymers thereof.
Preferably, the plastic substrate layer is a film, since thermoplastic films coated in accordance with preferred embodiments of the invention are particularly suitable for use as printable labels due to their excellent wet-scratch resistance, ink gloss and print image. The films can be clear, translucent, or opaque structures, having one or several layers. Examples of film substrates particularly suitable for use are found in U.S. Pat. Nos. 5,382,473, 5,380,587 and 5,194,324, which are herein incorporated by reference. One preferred cavitated structure is a biaxially oriented cavitated polypropylene/polybutylene terephthalate film, disclosed in U.S. Pat. No. 4,632,869, which is herein incorporated by reference. One preferred clear structure is a biaxally oriented coextruded polyolefin film having a skin layer comprising a random copolymer of ethylene and propylene containing from about 0.5% to 6% ethylene disclosed in U.S. Pat. No. 4,439,493 by Hein et al. incorporated herein by reference.
Total thickness of the plastic films of the present invention can range from 7.5 to 250 microns. Clear label films are generally 25 to 75 microns, with one embodiment being 35 to 55 microns. Cavitated (or opaque) and translucent film substrates for labels can have a thickness from 50 to 250 microns, with one embodiment being 60 to 100 microns. Films used for flexible packaging (clear or opaque) tend to be thinner than labels: 7.5 to 50 microns with 12 to 40 microns being preferred.
Depending upon the intended use, the plastic film can be coated on one or two sides with the coating applied by any means known in the art as a continuous film or as a pattern. In coated areas, the application rate of the coating can be between 0.05 and 5 grams/msi. Economics generally favor thinner coating layers; however, one might choose to use thicker layers of coating to impart stiffness, moisture or gas barrier, seal strength, or optical effects (e.g. color, opacity, or a matte finish) to the plastic film.
Cationically Stabilized Emulsion Polymer
Cationically stabilized emulsion polymer component used in the coating composition of the present invention comprises a combination of at least one polymerizable monomer which is uncharged or positively charged in an aqueous solution having a pH between 1 and 8, polymerized in the presence of at least one water-soluble polymer having a number-average molecular weight greater than 5000, which comprises a moiety selected from the group consisting of primary amines, secondary amines, tertiary amines and quaternary ammonium salts, with less than 5 weight per cent of the monomer units in the water-soluble polymer being comprised of copolymeric units derived from at least one member selected from the group consisting of carbohydrates, modified carbohydrates, polyamide-polyamine epichlorohydrin, and units having the following formula: 
wherein R1 is selected from the group consisting of H, C1 to C6 alkyl, and C1 to C6 acyl and R2 is selected from the group consisting of H, C1 to C6 alkyl, and the reaction product of epichlorohydrin with polyamides containing the following recurring groups:
xe2x80x94NH(CnH2nHN)xxe2x80x94COR3COxe2x80x94
where n and x are each 2 or more and R3 is a divalent organic radical of a dicarboxylic acid. An emulsion-forming component such as water or a hydrophilic material such as alcohols, glycol ethers, nonionic emulsifiers, or cationic emulsifiers having a number-average molecular weight that is  less than 5000 is added to provide an emulsion which can be applied to a substrate to produce a coated substrate.
On a dry basis, the polymerizable monomer can comprise:
i) 30 to 97 wt. %, preferably 55 to 95 wt. %, more preferably 80 to 93 wt. %, of at least one vinylic, non-acidic monomer which is uncharged or positively charged in an aqueous solution having a pH between 1 and 8; and
ii) 3 to 70 wt. %, preferably 5 to 45 wt. %, more preferably 7 to 20 wt. %, of at least one water-soluble polymeric compound having a number-average molecular weight greater than 5000, preferably greater than 7500, say, at least 9000 or even at least 10000, which comprises a moiety selected from the group consisting of primary amines, secondary amines, tertiary amines and quaternary ammonium salts.
The monomer can be selected from the group consisting of acrylic acid ester of C1 to C8 alcohol, methacrylic acid ester of C1 to C8 alcohol, acrylonitrile, methacrylonitrile, acrylamide, methacrylamide, N-substituted acrylamide, N-substituted methacrylamide, N-vinyl lactam, vinyl pyrrole, epoxy-functional vinyl compound, halogenated vinyl compound, vinyl monomer having a vinyl ester of an up to C6 saturated aliphatic monocarboxylic acid, vinyl ether, alkyl vinyl ketone, diester of alpha, beta-unsaturated dicarboxylic acid, butadiene, and styrene.
The C1 to C8 alcohol can be unsubstituted or it may comprise an additional moiety selected from the group consisting of halogen, hydroxyl, amino, aziridino, alkoxy, and epoxy. The epoxy-functional vinyl compound can be selected from the group consisting of 3,4-epoxy-1-butene, and 2-X-3,4-epoxy-1-butene, where X is selected from the group consisting of fluoride, chloride, and bromide.
A preferred water-soluble polymeric compound described in U.S. Pat. No. 3,719,629 to Martin et al., which is incorporated herein by reference, is an acidified aminoethylated interpolymer having pendant amino alkylate groups of the general formula: CO2(CHR1CHR2NH)nH, where R1 and R2 are selected from the group consisting of hydrogen and lower alkyl radicals comprising one to six carbon atoms, where the average value of n ranges from about 1.0 to 2.5.
Self-curing embodiments include those wherein at least one of the monomer(s) is an epoxy-functional monomer; and the water-soluble polymeric compound has a number-average molecular weight greater than 5000, preferably greater than 7500, and comprises a moiety selected from the group consisting of primary amines, secondary amines, and tertiary amines. The epoxy-functional monomer can be selected from the group consisting of glycidyl acrylate and glycidyl methacrylate (GMA). In a preferred embodiment, the self-curing, cationically stabilized emulsion polymer comprises at least one of said monomer(s) which is a nitrogen-containing monomer, e.g., a nitrogen-containing monomer selected from the group consisting of acrylonitrile and methacrylonitrile. Preferably, the water-soluble polymeric compound is present in an amount sufficient to stabilize an emulsion of the polymer and react with the epoxy-functional monomer when the emulsion is dried. The ratio of epoxy equivalents to reactive amine hydrogen equivalents in self-curing polymer can vary widely. However, the preferred ratio is in the range between 1:1 and 1:3, with a ratio between 1:1.5 and 1:2.5 being more preferred.
Coating Composition
The various cationically stabilized emulsion polymers described above are especially suitable for use in a coating composition for plastic film. Such coating composition preferably comprises at least one additive that provides an improved coating. Such an additive can be selected from the group consisting of: coating process-facilitating adjuvant, nonionic wax dispersion, cationic wax dispersion, nonionic slip additive, cationic slip additive, cationic colloidal silica, mineral filler, plastic pigment, adhesion promoter, cross-linking compound, curing catalyst, anti-static additive, and security taggant. Such additives, some of which are further discussed below, are known to those skilled in the art.
Coating process-facilitating adjuvants include defoamers, wetting agents, lubricants, and the like. For example, the coating composition when applied to the substrate layer may not xe2x80x9cwet outxe2x80x9d uniformly, especially when such materials are applied in very thin layers. As a result, the dry but as yet uncured liquid mixture may retract into droplets or xe2x80x9cislandsxe2x80x9d. Also, high-speed application of coatings can generate foam. Volatile additives are generally preferred over non-volatile defoamers and surfactant-like wetting aids. Ethylene glycol monohexyl ether (commercially available as Hexyl Cellosolve(trademark) from Union Carbide) facilitates wetting of the coating on the plastic substrate and helps to control foam. Other alcohols and glycol ethers like Dowanol(trademark) PM made by Dow Chemical Company can serve a similar function. Typically the wet coating formulation can comprise from 0.2% up to about 10% by weight of such volatile processing additives.
Nonionic or cationic wax emulsions can improve block resistance or 30 lower the coefficient of friction. Michemlube 156, produced by Michelman, Inc., is one of many suitable emulsions that are compatible with the cationic emulsion polymers disclosed in this invention. Such materials are generally unnecessary in cross-linked systems, but those skilled in the art know that such materials are important for balancing sealability properties with blocking resistance in coatings that contain little or no cross-linking. Depending upon the application, up to 15% of the dried film can comprise wax.
Slip additives besides wax include synthetic particulates like Nippon Shokubai""s Epostar(trademark) poly(methyl methacrylate) spheres that are 1 to 6 microns in diameter dispersed in water or alcohol containing a small amount of nonionic or cationic surfactant to aid dispersion. Equally useful are similarly dispersed non-meltable poly(monoalkylsiloxanes) having a mean particle size of about 0.5 to about 20 microns and a three-dimensional structure of siloxane linkages. Such materials are commercially available from Toshiba Silicone Co., Ltd and they are marketed under the trade name Tospearl(trademark).
Dupont Specialty Chemicals manufactures two grades of colloidal silica: Ludox(copyright) CL (average particle size 0.012 micron) and Ludox(copyright) CL-P (average particle size 0.022 micron). These materials can lower the coefficient of friction, especially when the flexible packing film or labels are required to move smoothly over heated surfaces. Moreover, these small particulates can help to improve the adhesion of many types of ink. Some applications might require that the dried film contain up to 60% colloidal silica. Other applications require none at all.
Examples of mineral fillers and pigments particularly suitable for use in cationic emulsions are found in U.S. Pat. No. 6,025,059 to McGee et al. Such additives could be expanded to include dyes and pigments to impart color to the coated film.
Useful adhesion promoters can be incorporated into the cationic emulsions to improve anchorage of the coating to certain substrates or to improve adhesion of a topcoat or ink to a substrate that has been coated with the formulated cationic polymer emulsion. Examples of adhesion promoters include, but are not limited to, chelated alkoxy titanates marketed under the trade name xe2x80x9cVertecxe2x80x9d are available from Synetix (a division of Imperial Chemical Industries PLC), Silquest(copyright) Silanes from Crompton Corporation, or derivatives of phosphinic acid, phosphonic acid, or phosphoric acid as described in U.S. Pat. No. 4,223,115 to Zaruda et al., incorporated herein by reference.
Adhesion of UV-curable coatings and inks to a substrate coated with cationic polymer emulsions of this invention can be improved by including polyfunctional acrylates resulting from the esterification of a polyol with (meth)acrylic acid or polyallyl derivatives as disclosed in a Republic of South Africa Patent Application 970523 (UCB), and incorporated herein by reference. Alternatively, one can accomplish the same purpose with epoxy acrylates formed from the reaction of a glycidyl ether of a member selected from the group consisting of polyethylene glycol and polypropylene glycol; and an unsaturated acid selected from the group consisting of acrylic acid and methacrylic acid. Suitable epoxy acrylates are available from Nagase Chemicals, Ltd., Tatsuno City, Hyogo, Japan under the tradename xe2x80x9cDenacol Acrylatexe2x80x9d UV or EB Curable Resin. Specific products include xe2x80x9cDM-811xe2x80x9d (epoxy methacrylate from (poly)ethylene glycol); xe2x80x9cDA-911xe2x80x9d (epoxy acrylate from (poly)propylene glycol); and xe2x80x9cDA-911Mxe2x80x9d (epoxy acrylate from (poly)propylene glycol). The presence of these non-volatile acrylate components can improve ink adhesion inasmuch as they add reactive double bonds to the coating composition, which can react with double bonds in UV-curable inks or lithographic inks. To hinder premature self-reaction during storage, one can incorporate a suitable stabilizer, e.g., one selected from the group consisting of methyl ether of hydroquinone and hydroquinone with methyl ether of hydroquinone being preferred.
The cationic polymer can be cross-linked with a cross-linking agent added after the polymer synthesis to improve solvent resistance of the coating or to attenuate properties such as hot tack, even if the polymer has functional groups to self-cross-link. For present purposes, IPA resistance can measure solvent resistance. Measurement of IPA resistance of the coated plastic film of the present invention can be carried out by rubbing a 70% IPA soaked swab about 4 by 4.5 centimeters available from Becton-Dickinson in a circular motion 20 times on a coated surface with light pressure using the index finger. Damage or whitening of the coating is then visually assessed after the IPA dries.
The cross-linking agent can be selected from the group consisting of polyfunctional aziridine, epoxy silane, polyfunctional epoxy, polyfunctional isocyanate, urea formaldehyde and melamine formaldehyde. Preferably, the cross-linking agent is selected from the group consisting of epoxy silane, polyfunctional epoxy, and melamine formaldehyde.
In some circumstances, the cross-linking agent is added with a cross-linking catalyst. Such catalysts are known to those skilled in the art and many are listed by Steiner et al. in U.S. Pat. No. 4,214,039 incorporated herein by reference. Preferred amine catalysts include Ancamine(copyright) K54 (Tris-2,4,6-[dimethylaminomethyl]phenol) and Imicure(copyright) EMI-24 (2-ethyl-4-methyl-1H-imadazole) manufactured by Air Products and Chemicals, Inc. These amine catalysts are also preferred for the self-curing cationic polymers of this invention. To avoid coagulation of the cationic emulsion, one should dilute these amine catalysts with water to about 1% before adding them to the emulsion. Alternatively, the amine catalysts can be diluted to about 10% solutions that have had the pH lowered to  less than 8 with a mineral acid like HCl or an organic compound like acetic acid. Acid catalysts are preferred for formaldehyde resins, preferably p-toluene sulfonic acid.
Some applications require the coated film to have anti-static properties. Many anti-static additives contain monomeric or polymeric quaternary ammonium salts. These additives are easily compatible with the cationic polymer emulsion of this invention. One such additive is diallyldimethyl ammonium chloride (261 RV manufactured by Calgon Corporation of Naperville, Ill.). Alternatively one can use combinations of nonionic surfactants and low-molecular-weight salts like lithium halides, choline chloride, lithium tetrafluoroborate, and other salts known in the art to impart anti-static properties to the coating.
Inclusion of security taggants in plastic films is useful in identifying counterfeited products or identifying the source of supply in product-liability cases. Any material that is insoluble, stable to conditions of use, and uniquely identifiable could be dispersed into the cationic coatings and applied to a plastic film. Micot Corporation manufactures small multi-colored chips that they market under the trade name Secutag(copyright). These insoluble, heat-resistant particles have particle sizes ranging from 5 to 125 microns. These inert materials are easily dispersed into the cationic emulsions of this invention. These markers are useful, because every customer has a unique color code built into at least four up to ten microscopic layers. Inclusion of a tiny amount of these materials into the coating can uniquely identify the source of the coating or the coated film.
Coated Plastic Film
The above-described coating composition can be applied to a plastic film. Such plastic film can be used in various applications including packaging and labeling. The plastic film comprises i) a plastic substrate and ii) any of the dried emulsion coating compositions of the invention described above. The plastic film of claim may comprise an ink print image on the side of said coating opposite from said plastic substrate. In one aspect, the plastic film comprises
A) a plastic substrate layer,
B) a primer layer containing a cationically stabilized emulsion polymer that comprises on a dry basis:
i) 30 to 97 wt. % of at least one vinylic, non-acidic monomer which is uncharged or positively charged in an aqueous solution having a pH between 1 and 8; and
ii) 3 to 70 wt. % of at least one water-soluble polymeric compound having a number-average molecular weight greater than 5000 which comprises a moiety selected from the group consisting of primary amines, secondary amines, tertiary amines and quaternary ammonium salts; with less than 5 weight percent, preferably less than 1 weight percent, of the monomer units in the water-soluble polymer being comprised of copolymeric units derived from at least one member of the group consisting of carbohydrates, modified carbohydrates, and units having the following formula: 
wherein R1 is selected from the group consisting of H, C1 to C6 alkyl, and C1 to C6 acyl and R2 is selected from the group consisting of H, C1 to C6 alkyl, and the reaction product of epichlorohydrin with polyamides containing the following recurring groups:
xe2x80x94NH(CnH2nHN)xxe2x80x94COR3COxe2x80x94
where n and x are each 2 or more and R3 is a divalent organic radical of a dicarboxylic acid; and
C) a coating layer which imparts properties to the film which properties are selected from moisture barrier, gas barrier, light barrier, printability, receipt of an image via a process selected from the group consisting of electronic, magnetic, thermal, and photographic processes, generation of an image via a process selected from the group consisting of electronic, magnetic, thermal, and photographic processes, carrying of anti-microbial agents, carrying of colorants, carrying of scents, water absorption, organic solvent absorption, release from pressure-sensitive adhesives, mar resistance, anti-static, conductivity, machinability, sealability, and adhesion.
The coating layer can be selected from vapor-deposited metals or metal oxides and any polymeric coatings known in the art, and can be in the form selected from at least one of the group consisting of aqueous solvent dispersions, aqueous solvent solutions, organic solvent dispersions and organic solvent solutions. Such coatings can comprise at least one member of the group consisting of acrylic, styrene acrylic, styrene butadiene, acrylonitrile-butadiene-styrene, poly(vinylidene chloride), poly(vinyl chloride), poly(vinyl alcohol), ethylene vinyl alcohol, ethylene-acrylic acid copolymer, polysilicates, silicones, polyurethane, and gelatin. Preferred metals and metal oxides include aluminum and aluminum oxide.
The coating of the present invention can be suitable for receipt of an ink image. The ink print image can be positioned on the side of the coating opposite from the plastic substrate layer. The finished plastic film can have a dry coating weight of at least 0.05 g/msi, with about 0.075 to 0.15 g/msi being preferred for applications requiring a clear film. In applications using filled coatings to create a matte or opaque finish, the finished plastic film can have a dry coating weight of at least 0.05 g/msi and up to 5 g/msi, with 0.5 g/msi up to 3 g/msi being preferred. Barrier and heat-seal coatings can have a dry coating weight of at least 0.3 g/msi and up to 5 g/msi.
Adhesive and Anti-Static Components
In order to provide printable labels, the non-print surface of the coated plastic substrate (i.e., the surface of the substrate opposite the coating) can be coated with various adhesives and have a releasable liner adhered thereon, or with anti-static coatings to improve application performance of coated substrates.
Primer Layer
In another alternative embodiment of the present invention, a primer or functional layer can be applied to the coating side of the plastic substrate prior to coating. Examples of the primer for thermoplastic materials include poly(ethyleneimine), which can be coextruded with or coated on the plastic substrate, and the epoxy coating at a low coating weight following the teaching of U.S. Pat. No. 4,214,039, to Steiner et al. Corona, plasma or flame treating can also be used with or instead of the primer. Functional layers can provide a barrier to gas and water vapor transmission, for example, or other beneficial properties of coatings listed earlier. Materials to be used as the functional layer include, but are not limited to, PVOH and PVdC.
Coating on Side Opposite of the Printable Layer
Coating may optionally be applied on the side opposite side of the substrate film from the printable layer to improve the adhesion of pressure sensitive adhesives or to improve blocking, etc.
Printing
The ink print image can be applied to the coated substrate using any known printing process. Examples include, but are not limited to, gravure, flexographic, lithographic, UV-screen, and intaglio printing processes. Similarly, the choice of inks to be used is variable.